1. Field of the Invention
This invention relates to an improved method and apparatus for evaluating focal characteristics and focal characteristic changes in ocular lenses of humans and animals in an in vitro culture.
The improved apparatus measures focal length, focal length variance and transparency of a given lens, and can be used to measure changes in these focal characteristics over time, for example in response to exposure to various irritants.
2. Description of the Prior Art
For many years, research interest has focussed on the examination of optical characteristics of lenses and the biology of aging of the lens. Meaningful examination of the lenses requires that the lens be examined intact so as to preserve the ocular integrity of the lens. In order to assess risk associated with lens exposure to various chemical products and environmental agents, an in vivo testing procedure was introduced, known as the Draize test. The procedure involved placing test material on the eyes of live animals (preferably albino rabbits), and evaluating ensuing ocular damage at varying subsequent intervals. The Draize test procedure has been criticized due to the subjective nature of the evaluation of tissue damage, and the uncertainty of the results. Also, since little is known about differences in chemical sensitivity as the eye ages, such irritability testing may be inaccurate. Additionally, animal rights advocates have been concerned with the pain and suffering endured by the test animals as a result of Draize testing. This extensive criticism has led to exploration of alternative methods which attempt to measure in vivo lens damage. Intact lenses cultured in vitro have been shown to maintain in vivo function of light passage and refraction. Therefore, in vitro examination of intact cultured lenses has been a focus of alternative methods of lens testing.
Lenses refract light from a point source some distance from the lens onto a focal plane behind the lens. A perfect lens will focus light directed from an infinite distance to a single point, defined as the focal point. If however a living lens is exposed to a toxicological agent, i.e. any substance which causes the surface of the lens or the interior of the lens to react to the substance, the shape, surface and/or interior quality of the lens will change. Because of the structure of the living lens, the effect of these disturbances varies at different locations across the width of the lens. The result is that the focal length (for example, the distance from the rear surface of the lens to the focal point), at each location across the lens will vary, i.e. there is increased focal length variability (essentially spherical aberration) of the lens. If the agent is removed from the lens and the lens is nurtured in growth medium, it is possible that the lens will attempt to repair itself. In some cases, given enough time, the lens will return to its original condition or close to it. By repeatedly measuring the focal length at points across the diameter of a lens as an agent is introduced and then removed, it is possible to quantify lens damage and recovery over time. Moreover, by using this type of measurement, comparative results can be obtained between different types and concentrations of agents.
In addition to focal length variability, it has also been shown that irritability of cultured lenses to chemical stimuli can be reliably evaluated by measuring lens scatter and thus lens transparency.
The use of a scanning laser to assess changes in lens characteristics is known. U.S. Pat. No. 4,832,486 (Gershon et al., including the present inventor) describes the use of an x-y table to scan the laser across the lens in two directions, the lens being positioned in a special container. The resulting light path images are analyzed via a complex process to determine focal length and focal length variability (spherical aberration).
Although this prior art method and apparatus have been effective in their study of ocular characteristics of intact lenses, there are certain deficiencies which have become evident through their use. Therefore, various improvements to the apparatus and the method to use same are desirable in order to allow for more accurate, reliable and less cumbersome measurements of lens focal characteristics and thus the study of lens pathology. In particular, it is highly desirable to avoid the complexities associated with scanning across the lens in two directions.
In the Gershon et al. patent, the lens optical axis is determined by locating the optical center position on the lens through which a laser beam incurs the minimal refraction. The optical center is located iteratively by scanning the laser through the lens at various positions on the x and y planes of the lens, moving the laser progressively closer in smaller steps until the beam passes through the lens without deviation. Two cameras are required for the analysis, i.e. one to look at the x-axis motion and one to look at the y-axis motion. Once the optical axis is determined, equivalent focal length is then measured by projecting the beam through the lens at various positions along the lens, in a plane passing through the optical axis. A more efficient means to determine the optical axis than this multi-step process is required.
Equivalent focal length is typically measured as the distance from the principal plane within the lens (the intercept of the incoming beam with the exiting beam) to the intercept of the beam to the optical axis. Any variation in focal length at different positions along the lens is influenced predominantly by spherical aberration, however the presence of coma in the lens is also a factor. In order to better determine focal length variability caused by spherical aberration rather than coma, a more reliable measurement is required.
Additionally, the Gershon et al. patent used a laser in which entails the beam which is slightly oval in cross section, making it difficult to equate video beam thresholds (width and brightness) in the two directions. An improved method involving unidirectional laser movement is preferred.
In addition to measuring refractive conditions of the lens, such scanning laser systems attempt to measure lens scatter (transparency of the lens) for each laser position. However, scatter measurements have proven to be difficult to interpret in comparison to focal measurements and therefore have not been utilized to determine lens health. An effective means to present and evaluate lens transparency information is required.
Improvements to the lens containers are also desirable. The Gershon et al. lens container, described in the above-mentioned patent and more thoroughly in U.S. Pat. No. 4,865,985 also by Gershon et al., has shown some problems with leakage of the medium in which the lens must sit to be scanned by the lens. The prior container also did not permit observation of the rear surface of the lens. Therefore an improved lens container is also desired as part of the overall system.
It is an object of the invention to provide an improved method and apparatus for determining lens focal length, evaluation of spherical aberration of the lens, and measurement and analysis of lens transparency, and a method of comparison of these measurements over time.
In particular, it is an object of the invention to avoid the use of an x-y table, and to avoid the need for analysing data from two cameras.
In the invention, therefore, although a second camera is used for the purpose of roughly aligning the scanning laser with the lens axis, the laser is scanned in only one dimension, and data from scanning is collected from only one camera. This greatly simplifies the apparatus and the method, and speeds up the process considerably.
In the invention, a vertebrate eye lens sits horizontally in a culture medium within a transparent lens container within an area for viewing the lens. The laser is directed upwardly through the lens, for scanning across the x direction. With the aid of an alignment camera facing the lens in the x direction, the position of the platform on which the lens container is carried is manually adjusted in the y direction so that the laser is aligned approximately with the lens axis in the y direction. An analysis camera is directed at the lens normal to the lens optical axis, for viewing the lens, and the path of the laser beam after it exits the lens. The laser beam is translated in the x direction, preferably by moving a mirror on a carriage, to direct the laser beam through the container and the lens, parallel to the lens axis at a plurality of locations along the x axis. The analysis camera captures the images of the beams at the plurality of locations, for computer analysis.
A means for rotating the lens and lens container in relation to the translational path of the laser beam is also provided so that multiple axes of the lens can be scanned, if desired. A means for recording data received from the analysis camera image and a means for analysis of that recorded data for determination of focal length, and lens transparency is also provided.
In contrast to the previously-mentioned Gershon et al. patent, the laser in the invention is scanned in one direction only. The lens is manually centered relative to the laser beam, preferably with the aid of an alignment camera.
As the laser beam path is analysed, the invention preferably also provides an effective simultaneous measurement of light intensity of each refracted beam, and thus an indicator of lens scatter (transparency) at various positions along the lens.
In the analysis of the images captured by the analysis camera, focal length at various positions along the lens is calculated using back vertex distance, that being the distance from the back vertex position to the focal point. Back vertex distance, i.e. the distance from the rear surface of the lens to the focal point, has been found by the inventor to be a preferable measurement over equivalent focal length since any discrepancy in back vertex distance is due solely to spherical aberration, rather than possibly due to coma.
To achieve first the desired alignment and then the desired measurements, the lens laser interface must be visible to the cameras. Thus the container in which the lens sits presents the lens to the cameras such that back vertex of the lens and the lens focal area (the area between the lens focal point and the lens) are viewable by the cameras, which was not the case with the previous Gershon et al. container.
The apparatus therefore uses an upright lens container, having transparent side walls extending upwardly from a base unit, the base unit having a transparent central portion at its bottom. The lens is supported upon a lens carrier unit and a lens holding washer, such that the lens will be exposed to a laser beam projected through this central portion, through the bottom of the container. The container base unit is attached to the sidewalls by means of a detachable liquid-tight seal. The container is oriented such that the laser beam can be sent through the bottom transparent portion and then the lens, parallel to the lens optical axis.
Within the container, the lens support unit preferably comprises corner posts extending upwardly from the base unit, for support of the lens holding washer. The lens holding washer is designed to sit on the corner posts, with the lens being supported peripherally by the lens holding washer. The side walls must extend at least as high as the largest focal area of subject lenses so that the entire focal area will be contained within the container and thus the relevant laser beam path can be viewed in the culture medium by the cameras.
The side walls of the container are flat-surfaced, in order to minimize the refraction effects while looking through the side of the container. The base of the container is sized and configured to sit on a sliding platform which sits upon the laser scanner apparatus.
The invention also provides a method for evaluating focal length, spherical aberration and lens transparency using the above apparatus. An intact vertebrate lens is positioned in a culture medium within the transparent lens container. Preferably, the lens and the container are positioned for viewing by two digital cameras, namely an alignment camera for aligning the lens relative to the laser beam, and an analysis camera at ninety degrees to the alignment camera, for viewing the lens and laser beam path and intensity. The laser beam is projected from below the container, through the central transparent portion. The alignment camera displays an image of the lens in the x direction, and the platform position is then manually moved in the y direction via an alignment knob, to approximately align the laser beam with the lens axis (as observed by straight-through passage of the beam). The laser beam is then scanned across lens in the x direction, at a plurality of positions along the lens. The analysis camera views and captures the intensity and directional path of the laser beam as it passes through the culture medium, locating the back vertex location of the lens (by determining the point of maximum brightness) and the subsequent path of the laser beam, at the various positions along the lens. The information captured by the analysis camera is analysed to determine back vertex distance, focal length, focal length variance and transparency of the lens.
As in the prior art, a stimulus may be applied to the lens in the culture medium, and the above steps may be repeated after some elapsed time period, with the focal length, focal length variance (spherical aberration) and transparency of the lens before and after that period of time being compared, so as to determine the irritancy of the lens in response to the stimulus.
The invention further provides computer software which processes the images captured by the analysis camera to determine back vertex location at various locations on the lens, focal length at each location relative to the back vertex, the average focal length for the lens and standard deviation and error of the lens focal length. Similar results for the relative intensity of the beam are also calculated by the software program.
Further features of the invention will be described or will become apparent in the course of the following detailed description.